1. Field of the Invention
The present invention relates to a starter motor equipped with a shock absorbing device or an excessive torque absorbing device.
2. Description of the Related Art
There is well known a starter motor equipped with a shock absorbing device (or an excessive torque absorbing device). The shock absorbing device is a multiple disk type, which is composed mainly of a plurality of disks which are stacked. For example, Japanese patent laid open publication NO. 2005-113816 has disclosed such a starter motor.
FIG. 7 shows a cross section of a conventional shock absorbing device of a multiple disk type assembled into a conventional starter motor.
As shown in FIG. 7, the shock absorbing device is composed mainly of a cylindrical casing 100 having one bottom part, a plurality of rotatable disks 110, a plurality of fixed disks 120, and a disk spring 130. The rotatable disks 110 are rotatably placed in the inner periphery of the cylindrical casing 100. The fixed disks 120 are fixed in the cylindrical casing 100. The rotatable disks 110 form an internal gear of a speed deceleration device (or a planetary gear speed reduction device). The rotatable disks 110 and the fixed disks 120 are alternately placed along the thickness direction (or along the axial direction) in the cylindrical casing 100.
The rotatable disks 110 in the shock absorbing device rotate against the frictional force which is generated between the rotatable disks 110 and the fixed disks 120. The rotation of the rotatable disks 110 absorbs an excess torque or force applied to the internal gear from outside when an internal combustion engine starts to rotate.
However, the conventional shock absorbing device in the starter motor has the above structure in which the disk spring 130 is placed on the opposite surface of the bottom part of the cylindrical casing 100. That is, as shown in FIG. 7, the disk spring 130 is the upper side of the disk stack structure, which is far apart from the bottom part of the cylindrical casing 100. In other words, the disk stack structure is placed between the bottom part of the cylindrical casing 100 and the disk spring 130. This structure of the conventional shock absorbing device requires to form the inner diameter of the disk spring 130 to being smaller than the diameter of the tooth bottom of the internal gear (as composed of the rotatable disks 110) in order to avoid any interference between the disk spring 130 and a planetary gear which is mated with the internal gear. This structure reduces the width of the disk spring 130. In other words, because this structure decreases the ratio (outer diameter/inner diameter) of the outer diameter and the inner diameter of the disk spring 130, the force generated by the deflection of the disk spring 130 increases, and as a result, the durability of the disk spring 130 decreases.
When the end part at the opening side of the cylindrical casing 100 is caulked toward its inside direction in order to bend the disk spring 130, the sloped part (as the sloped surface) of the disk spring 130 is forcedly pushed to the end part at the opening side of the cylindrical casing 100. This introduces a possibility of inclining the disk spring 130 when the cylindrical casing 100 is caulked. On caulking, because the load applied to the disk spring 130 becomes unstable, the shock absorbing capability of the shock absorbing device becomes varied. Furthermore, caulking the end part at the opening side of the cylindrical casing 100 toward its inside direction to bend the disk spring 130 requires to fix both the disk spring 130 and the cylindrical casing 100. Thus, there is much left to improve the caulking process in the starter motor assembling work.